EP-537 044 describes a bottom end-piece for such an assembly. That end-piece comprises a horizontal wall which is provided with feet for support on the lower plate of a nuclear reactor core. The elements for connecting the bottom end-piece to the top end-piece are constituted by guide tubes. Those guide tubes are fixed to the horizontal wall of the end-piece. The horizontal wall comprises reinforcement ribs under the lower surface thereof. In each zone of the horizontal wall delimited between the reinforcement ribs, holes for the passage of coolant water are provided so that the horizontal wall constitutes an anti-debris filter.
The coolant water flows in the core of the reactor vertically in an upward direction. More precisely, the water is introduced into the core through the lower core plate, then passes through the bottom end-piece by way of the above-mentioned holes before coming into contact with the outer surfaces of the fuel rods.
The water flows in the core at a very high ascending rate.
It has been found, during operation of the core, that the fuel rods, and in particular the lower ends thereof, were subjected to vibrations which are liable to damage them.
In greater detail, phenomena involving friction or “fretting” are liable to occur in particular between the lower grid of the support skeleton and the outer claddings of the fuel rods.
These friction phenomena may lead to damage to outer claddings which may bring about a release of fission gas or product in the water of the primary circuit.